Amazon will be selling its Kindle 2 for just $89 on Friday.

If you've been waiting for a sub-$100 e-reader, the wait is over Friday.

Amazon announced on its Facebook page that it will be selling the second-generation Kindle 2 for $89 on Friday. The offer is one of Amazon's "lightning deals," which begin at 9 a.m. PST on Friday. Lightning deals sell in limited quantities, so you'll need to be ready to buy as soon as the $89 Kindle 2 becomes available.

Of course, Amazon released a third-generation Kindle in August, with 50% better contrast thanks to a new generation of E-Ink. The new model is also lighter, smaller, and faster at refreshing pages than its predecessor. It also has twice the memory with 4 GB of storage, which amounts to 3,500 books. The Kindle 3 costs $139 with Wi-Fi and $189 with Wi-Fi and 3G.

E-readers are already pretty thin and light to begin with, but as PC World's full review notes, the Kindle 3 is the first reader to truly feel comfortable in one hand. It also has minor improvements, such as rearranged navigation buttons, over the Kindle 2.

Is the $89 Kindle 2 worth pursuing? It depends on your budget. It's certainly the cheapest way to get 3G coverage in an e-reader (the Kindle 2 offers built-in 3G access from Spring, at no extra cost), but maybe you don't mind downloading all of your books over Wi-Fi. If that's the case, dropping an extra $50 for a better product could be worthwhile, and you won't have to stress about beating the online rush